One motor finger mechanism

ABSTRACT

A mechanical finger comprises a plurality of phalanges coupled to a single actuator using a kinematic linkage and a differential linkage arranged in parallel. The mechanical finger is capable of exhibiting consistent predictable motion when moving in free space or when contacting an object at the fingertip, and of curling in order to conform to an object when the contact is at other locations on the finger.

This application claims the benefit of U.S. provisional patentapplication No. 61/286,345, filed Dec. 14, 2009, which is incorporatedherein by reference for all purposes.

This invention was made with government support under Contract No.N66001-06-C8005, awarded by Defense Advanced Research Projects Agency.The government has certain rights in the invention.

BACKGROUND

A “mechanical finger” refers to an elongated, articulating, mechanicalappendage. Like a human finger, a mechanical finger has one end joinedto a structure that acts as a base and an opposite end that is notanchored or connected. A mechanical finger used for grasping typicallyhas two or more rigid sections, and preferably at least three, connectedend to end by articulating joints. Terminology used to describe theanatomy of a human finger is used to describe a mechanical finger. As inthe human finger, each section of the finger is referred to as a“phalanx.” A finger extends from a base and is comprised of at leasttwo, and preferably three, phalanges joined end to end by pivoting orarticulating joints. A first articulating joint joins a proximal phalanxto a base, such as a palm of a hand. A second articulating joint joinsthe proximal phalanx to an intermediate or middle phalanx, and a thirdarticulating joint joins the intermediate phalanx to a distal phalanx.The first joint is referred to as the metacarpophalangeal (MCP) joint,the second as the proximal interphalangeal (PIP) joint, and the third asthe distal interphalengeal (DIP) joint.

In a mechanical finger, the phalanges are coupled to one or more motorsto cause flexion and extension of the finger. When using a kinematicmechanism for coupling a single motor to the phalanges, the position ofthe actuator fully determines the position of the joints, but the torqueat each joint is unknown. With a differential mechanism, the torque atthe actuator determines the torque at each of the driven joints, butneither the velocity nor the position of the individual joints arespecified by the actuator velocity or position alone. A kinematicmechanism produces consistent, predictable motion of the finger joints,but it does not allow the finger to curl around an object. Differentialmechanisms allow curling and grasping, but often deviate from thedesired motion due to forces at the fingertip, causing buckling, or dueto friction in the joints, causing undesirable curling behavior when notconforming.

SUMMARY

According to one aspect of an exemplary embodiment of a mechanicalfinger comprising at least two phalanges driven by a single actuator,and a differential transmits torque in parallel from the actuator to theMCP joint and the PIP joint.

According to another aspect, the mechanical finger further includes avariable stop that limits rotation of the PIP joint based on the angleof rotation of the MCP joint. Such a mechanical finger is capable ofexhibiting consistent predictable motion when moving in free space orwhen contacting an object at the fingertip, and curling in order toconform to an object when the contact is at other locations on thefinger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a mechanical finger driven by asingle actuator.

FIG. 1B is a schematic illustration of an alternate embodiment of amechanical finger driven by a single actuator.

FIG. 1C is a schematic illustration of an alternate embodiment of amechanical finger driven by a single actuator.

FIG. 1D is a schematic illustration of an alternate embodiment of amechanical finger driven by a single actuator.

FIG. 1E is a schematic illustration of an alternate embodiment of amechanical finger driven by a single actuator.

FIG. 2 is a perspective view of an example of a prosthetic finger,partially constructed and without a covering, embodying a couplingmechanism according to the principles of the mechanical finger of FIG.1.

FIG. 3 is an exploded view of the prosthetic finger of FIG. 2.

FIG. 4A is a side view, rendered with perspective, of proximal andmedial phalanges of the prosthetic finger of FIG. 2, which is onlypartially constructed to reveal a differential linkage.

FIG. 4B is a side view, not rendered with perspective, of the partiallyconstructed proximal and medial phalanges of the prosthetic finger ofFIG. 2, in an extended position.

FIG. 4C is a side view, not rendered with perspective, of the partiallyconstructed proximal and medial phalanges of the prosthetic finger ofFIG. 4B, in a fully flexed position.

FIG. 5A is a side view, rendered with perspective, of proximal andmedial phalanges of an alternate embodiment of a prosthetic finger thatis partially constructed to reveal a differential linkage.

FIG. 5B is a side view, not rendered with perspective, of the partiallyconstructed proximal and medial phalanges of an alternate embodiment ofthe prosthetic finger of FIG. 2, in an extended position.

FIG. 5C is a side view, not rendered with perspective, of the partiallyconstructed proximal and medial phalanges of an alternate embodiment ofthe prosthetic finger of FIG. 5B, in a fully flexed position.

FIG. 6 is a side view, rendered in perspective of proximal and medialphalanges of alternate embodiment of a mechanical partially constructedto reveal a differential linkage.

FIG. 7 is a side, perspective view of the partially constructedprosthetic finger of FIG. 2, with certain elements removed to reveal alinkage.

FIG. 8A is a side, non-perspective view of the prosthetic finger of FIG.2, with several parts removed to illustrate a stop linkage.

FIG. 8B is a perspective view of FIG. 7B.

DETAILED DESCRIPTION

In the following description of a mechanical finger, like numbers referto like parts.

FIGS. 1A-1E schematically illustrate several alternative embodiments ofmechanisms for driving a mechanical finger 100 using a single motor. Themechanism combines a differential, a kinematic linkage and a PIP linkagefor coupling the torque and position of a drive output to a mechanicalfinger 100 having at least two sections in order to control its flexionand extension in a manner that permits it to be used in connection withgrasping or other applications in which a curling action is desirable.Such applications include, but are not limited to, robotic hands andprosthetic hands.

The illustrated examples of mechanical finger 100 comprise at least aproximal phalanx 102, a medial or middle phalanx 104, and, in theembodiments of FIGS. 1A to 1E, a distal phalanx 106. “Phalanx” refers toan elongated, rigid section of the finger, and “phalanges” to multiplesections of the finger. The phalanges are sometimes also referred toherein as first, second and third sections, respectively, of themechanical finger. Articulating joints, which are not expresslyindicated in the figure, permit joined phalanges to pivot with respectto each other around an axis of the joint. The X-axis 108 of the figurerepresents the angle of extension and flexion of the phalanges relativeto each other and to a reference ground 110. A greater angle indicatesflexion of the finger and a smaller angle indicates extension of thefinger. The length of arrow 112 represents the angle, designated by thevariable Θ_(PF) between the proximal phalanx 102 and a ground 110.Similarly, the lengths of arrows 114 and 116 represent the relativeangles between the proximal phalanx and the middle phalanx, and betweenthe middle phalanx and the distal phalanx, respectively. These anglesare designated in the figure by the variables Θ_(MF) and Θ_(DF),respectively.

The angular position and torque transmitted by an output of a singleactuator or drive, which output is represented by line 118, controls theflexion and extension of the finger. Any type of suitable motor canpower the actuator or drive. The type of the motor will depend on theapplication. The angular position of the output is represented by line120 and is designated by the variable Θ_(m). Torque applied to an objectby a joint is represented as a linear force in the figure. The torquedelivered by the output of the drive is represented by line 122.Variable T_(m) represents the magnitude of the torque from a motorconnected to the drive. Note that the motor is not expressly illustratedin the figures. Torque on the metacarpophalangeal (MCP) joint (notshown), designated T_(mcp), which is generated by force applied to theproximal phalanx, is represented by line 103. Similarly, torque on theproximal interphalangeal (PIP) joint (not shown) is designated T_(pip)and is represented by line 105. Torque on the distal interphalangeal(DIP) joint (not shown) is designated T_(pip) and is represented by line105.

A hybrid mechanism comprising a kinematic linkage and differentialenables conformal grasping by the finger due to the differential, but atthe same time curling behavior can be precisely defined duringapplication of forces to the distal phalanx only. In the examplesillustrated by the schematics of FIG. 1A-1E, a differential 124 coupledto ground 110 applies the torque T_(m) from the motor to the proximalphalanx 102. The differential also applies the torque to linkage 130 inthe embodiments of FIGS. 1A, 1B, 1E or to medial phalanx 104 in thetwo-phalanx embodiment of FIG. 1C, or to a second differential 125 inthe embodiment of FIG. 1D. The differential 124 couples the drive outputwith the MCP joint and the PIP joint. Thus, the drive applies torque toboth the PIP and MCP joints in the embodiments of FIGS. 1A-1C and 1E. Inthe embodiment of FIG. 1D, the combination of differential 124 anddifferential 125 applies torque applied to the MCP, PIP and DIP joints.

Linkage 130 in FIGS. 1A, 1B, and 1E functions as a kinematic linkage,coupling the motion of PIP and DIP joints through an algebraicrelationship. Linkage 130 couples the PIP and DIP joints (not shown), sothat both joints rotate together, in a fixed relationship, resulting inthe medial and distal phalanges curling together in a natural curlingmotion. Movement of link 130 relative to the proximal phalanx 102 causesthe middle phalanx to rotate about the PIP joint (not shown), and thedistal phalanx to rotate with respect to the medial phalanx around theDIP joint (not shown). This coupled curling relative to the proximalphalanx 102 occurs even while motion of proximal phalanx 102 is blocked,such as when conformal grasping is occurring

As shown in the embodiment illustrated only in FIG. 1B, the linkage 124may, optionally, include a compliant element 128, in series with ground,represented in the figure by spring 128. The compliant element is, forexample, comprised of an elastic element that generates a spring force.The spring provides compliance for series elasticity and shockmitigation by allowing linkage 124 to stretch a little when forces areapplied to it. Elasticity and shock mitigation or dampening can bedesirable in certain applications, such as prosthetics. Movement of thelinkage 130 relative to the proximal phalanx 102, such as during curlingwhen the proximal phalanx 102 is blocked, also results in compression ofa compliant member represented in the figure by a spring 132 coupledbetween the proximal phalanx and the link 130. The spring acts to extendthe PIP joint.

Referring only to FIGS. 1A-1D, in each of the illustrated examples alinkage 126 adjusts the position of stop 134 based on rotation of theMCP joint. Stop 134 limits the range of motion of the PIP joint. Thelinkage sets the position of the hard stop based on the degree ofrotation of the MCP joint from ground. Stopping rotation of the PIPjoint limits extension of the medial phalanx, as well as the distalphalanx, beyond a predetermined angle relative to the proximal phalanx.The angle of rotation of the MCP joint is represented in the figure asthe distance between ground 110 and the proximal phalanx 102. The angleof the PIP joint relative to the phalanx is indicated by the length ofline 114 in the figure. The stop rotates with respect to the PIP jointas the MCP joint rotates, and thus it depends on the angle of the MCPjoint. When the proximal phalanges motion is not blocked, the stoplinkage 126 enforces natural, simultaneous curling of all three joints,the MCP, PIP and DIP joints. Linkage 126 also enables the finger toresist forces on the distal phalanx without the differential allowingthe PIP and DIP joints to straighten and the MCP joint to flex. Despitethe system having a differential, the posture of all three joints canthus remain fixed (not against stops) irrespective of the magnitude of asingle external force applied to the distal phalanx.

Because of the use of a differential linkage to couple torque from thedrive to the MCP and PIP joints, the positions of the MCP and PIP jointsare not fully determined by the position of the drive. For any givenposition of the drive output, the finger mechanism has one free motionavailable, which is an extension of the proximal phalanx and a flexingof the PIP and DIP joints. Preferably, linkage dimensions and momentarms are chosen so that external forces applied to the finger distal toa point near the fingertip act to straighten the finger, and forcesapplied proximal to this point act to curl the finger. The point atwhich the behavior changes from straightening to curling is referencedas the “focal point” of the differential. For external forces that actproximal to the focal point, the MCP joint will extend and the PIP jointwill flex.

Referring now to FIGS. 1A-1E, the linkage 126 is also used to move theendpoint for return spring 132. The return spring 132 acts to straightenthe finger and to keep the mechanism pushed over to one side of thisfree range of motion. In the absence of any external forces pushing onthe finger, the return spring makes the finger act as though thedifferential 124 is not present. The return spring can also provide someresistance to curling of the fingers when forces are applied to thedorsal side of the finger. Any compliance in the differential 124 willresult in some motion, but this will occur in all three joints and isnot due to the differential coupling.

As illustrated by the embodiment of FIG. 1E, adjustable stop 134 for thePIP joint may be omitted for an application not requiring it, or inwhich it is desirable not to have it. In this example, the linkage 126controls only the position of the end point of the PIP joint returnspring 132. The linkage 126 thus becomes a spring centering linkage.

Referring now only to FIG. 1D, this embodiment of a mechanical fingerincludes a differential 125 comprising differential linkage 136 in placeof a kinematic linkage. The differential couples the medial and distalphalanges using a differential relationship. This embodiment alsooptionally includes an adjustable stop 138 for the DIP joint and returnspring 140 for placing a torque on the DIP joint that tends to extendthe distal phalanx relative to the medial phalanx. Linkage 142 isconnected to proximal phalanx 102 and adjusts the position of DIP stop138 based on the angle of rotation of the PIP joint. It also sets theendpoint of return spring 140.

FIGS. 2, 3, 4A-4C, 5A-5C, 6, 7 and 8A-B illustrate various aspects of anexemplary embodiments of mechanical finger 100 for use in a prostheticapplication. The prosthesis comprises at least one prosthetic finger200. The prosthesis may also include, depending on the needs of thepatient, a prosthetic hand, comprising a prosthetic palm to which themechanical finger is attached, and a prosthetic arm, to which theprosthetic hand is attached. Only the internal structure of theprosthetic finger is illustrated in the figures.

Prosthetic finger 200 is comprised of proximal phalanx 202, medialphalanx 204, and distal phalanx 206. Distal phalanx 206 has been omittedfrom FIGS. 4A-4F for purposes of illustration. Metacarpophalangeal (MCP)joint 208 connects the finger to a base element, for example, anartificial palm or hand, which is not shown. Proximal interphalangeal(PIP) joint 210 joins the proximal and medial phalanges. Distalinterphalangeal (DIP) joint 212 joins the medial and distal phalanges.

In the embodiment shown in FIGS. 3 and 4A-4C, the proximal phalanx 202houses a differential linkage comprised of a connecting rod 214, a pivotlink 216, and another connecting rod 218. Connecting rod 214 is joinedby pin 220 to an arm extending from drive output 222, and thus connectsthe output drive to one end of the pivot link 216. Although not shown, amotor—a stepper motor, for example—located in the base element rotates adrive input, which in this example is pin 223, which in turn rotates thedrive output. Drive output 222 is fixed to the pin 223. Pin 221 joinsthe connecting rod to the pivot link. Connecting rod 218 connects theother end of the pivot link to plates 228 a and 228 b of the medialphalanx 204. Pin 224 joins the pivot link to the connecting rod 218, andpin 226 joins the connecting rod to the plates 228a and 228b, whichcomprise the primary structural elements for medial phalanx 204.

The midpoint of the pivot link is fixed by pin 230 to plates 232 a and232 b. The pivot link will rotate within the proximal phalanx, about theaxis of pin 230, as indicated by comparing FIGS. 4B and 4C, when thedrive output rotates. During flexion, rotation of the drive output 222pulls the connecting rod 214, which pulls on the pivot link 216, whichpulls on a second connecting rod 218, which pulls on plates 228 a and228 b of the medial phalanx.

In an alternate embodiment shown in FIGS. 5A-5C, the pivot link 216(FIGS. 4A-4C) is replaced by an in series compliant element for givingthe finger compliance for series elasticity and shock mitigation. Inthis example, the compliant element comprises spring 217. Except for theadded compliance and elasticity provided by the spring, the differentialwith spring performs in a substantially similar manner as the pivot link216. In another alternate embodiment shown in FIG. 6, the pivot link 216and the connecting rods 214 and 218 are replaced with a linkagecomprising a single connecting rod 219 that is connected by pins 220 and226 to the drive housing 222 and plate 228 b of the medial phalanx 204.As can be seen in the figure, the connecting rod must extend beyond theenvelope of the proximal phalanx 204.

In each of the embodiments shown in FIGS. 2 to 8B, plates 228 a and 228b are the primary structural elements of medial phalanx 204. Plates 232a and 232 b are the primary structural elements comprising the proximalphalanx 202. The differential linkage of FIGS. 4A-4C and 5A-5C describedabove is housed between the plates. To these plates can be attachedshells to give the proximal phalanx its desired exterior shape in theparticular prosthetic or other application.

The pins used to join components in the differential linkage, as well asin other linkages described below, permit relative rotation of thejoints that are joined. The location of pin 226 is eccentric to the axisof the PIP joint to form a moment arm. The axis of the PIP joint isdefined by pin 236, which pivotally connects the clevis formed by plates232 a and 232 b of the proximal phalanx with plates 228 a and 228 b ofthe medial phalanx. For a given rotation of the drive output, either theMCP joint or the PIP joint can rotate. Rotation of the drive output notonly applies torque to the MCP joint by causing the pivot link to pushagainst pin 230, but it also rotates the link, causing the other part ofthe link to transmit a force that is applied to pin 226. Even if theproximal phalanx is blocked, the link will nevertheless pivot and applytorque to the PIP joint. Thus, torque from the drive is applied to boththe MCP joint and the PIP joint.

Referring now to FIGS. 2, 3, and 7, the medial phalange 204 houses akinematic linkage for coupling rotation of the PIP joint to the DIPjoint so that both curl simultaneously. The kinematic linkage comprisesa connecting rod 238 that spans between the proximal phalanx 202 and thedistal phalanx 206. Pin 240 at a proximal end of the connecting rodengages hole 242 on plate 232 b of the proximal phalanx. Pin 244 on thedistal end of the connecting rod engages hole 246 in the distal phalanx.The distal phalanx is linked to the medial phalanx by a hinge formed bypins 248 a and 248 b. These pins cooperate respectively, with a hole onplate 228 a and a hole on plate 228 b of the medial phalanx, and withholes 250 a and 250 b on opposite forks of a clevis extending from ashell forming distal phalanx 252. Although in this embodiment thelinkage is comprised of a single connecting rod, it could comprisemultiple links Furthermore, a differential could be substituted for thekinematic linkage, as described in connection with FIG. 1D.

Referring now only to FIGS. 2, 3, 8A and 8B, the mechanical finger 200includes, in this embodiment, fixed stop 253 that stops rotation of thePIP joint to prevent hyperextension of the medial phalanx. In thisembodiment, a movable PIP stop part 254 rotates on the same axis as thePIP joint to reduce the permitted range of motion of the medial phalanxby limiting further rotation of the PIP joint based on the degree offlexion of the MCP joint. The centerline of pin 236 defines the axis ofrotation. The PIP stop part includes a stop portion 255 that interfereswith 257 of plate 228 a of the medial phalanx to prevent the medialphalanx from extending. The position of the PIP stop part 254 is basedon the degree of rotation of the MCP joint, and is accomplished in thisembodiment by a linkage comprising connecting rod 260 between a housing256 for a drive (not shown) and PIP stop part 254. The linkage may alsobe implemented using multiple links. A pin connects the distal end ofconnecting rod 260 to arm portion 264 of the PIP stop part 254. Theproximal end of connecting rod 260 is connected by another pin to thedrive housing 256. As the MCP joint rotates due to flexion of theproximal phalanx 202, the connecting rod pulls on the arm 264, causingthe PIP stop part to rotate in the same direction.

With the PIP-stop linkage, the medial phalanx 204 is stopped either bythe fixed stop 253 on the proximal phalanx when the proximal phalanx isfully extended, or by the movable stop of PIP-stop part 254 when the MCPjoint is rotated during flexion of the proximal phalanx. If the MCPjoint rotates, then the PIP joint is forced to rotate as well by thePIP-stop part. During free motion, or when forces are applied to thefingertip, movement of the PIP-stop part helps to produce predictablecurling like a fully kinematic mechanism.

In this embodiment, the rotational position of the PIP-stop part 254also controls the endpoint 270 of the return spring 266. This spring,which is normally compressed, has the effect of extending the medialphalanx, thus pushing the PIP joint against the PIP-stop. If no externalforces act on the finger, the force generated by the spring causes themotion of the finger joints to be controlled by the PIP-stop. If,however, an object blocks the motion of the proximal phalanx, then thedifferential linkage continues applying torque to the PIP joint, causingPIP and DIP joints to curl and further compressing the return spring.

The kinematic linkage for controlling the position of the PIP stop basedon the motion of the MCP joint could also be used to limit or affect themotion of the PIP and DIP joints in other ways. For example, the PIPstop can be removed, permitting the linkage to be used for controllingthe end point of the return spring without limiting the motion of thePIP joint.

Although not necessary for operation of the finger as described above,joint positions can be measured using potentiometers coupled with thejoints and feedback to a controller for the drive motor in order todrive the finger to desired position, subject to the limitations ofbeing able to do so caused by the differential. Similarly, strain gaugescan be placed on, for example, the drive housing 256 to measure torqueon the finger and feed the measured torque back to a controller tochange the impedance of the finger.

Although the particular components forming the linkages and thephalanges illustrated in FIGS. 2-7 have advantages when used in aprosthetic application, the structures are intended to be illustrativeonly of the linkage mechanisms illustrated by FIG. 1. These componentscan be adapted or substituted for when implementing a differentialmechanism in parallel with a kinematic mechanism in accordance withFIG. 1. For example, linkages may be replaced with belts or cables orother passive mechanical mechanisms to achieve the same general purpose.Although it is common to use linkages for kinematic mechanisms andcables for differential mechanisms, but either type can be used foreither purpose. In addition to being implemented as a linkage, asexemplified by FIGS. 2-8B, the differentials described above may also beimplemented using a belt or cable, for example one linking the driveoutput to a drum or pulley at the PIP joint, a gear train, or a toggle.

Furthermore, applications in which a mechanical finger in accordancewith FIGS. 1A-1E can be used include any type of application involvinggrasping, and include many different types of robotic applications thatare not limited to those attempting to mimic a human hand or prostheticapplications. For instance, an anthropomorphic grip may have benefits inmany diverse or unstructured or unforeseen contexts just as human handsare so successfully versatile, including industrial grippers, rovers ormobile robots, entertainment, home robots, surgery or minimally invasivesurgery, massage, patient transfer or stabilization, and many others.

The foregoing description is of exemplary and preferred embodiments. Theinvention, as defined by the appended claims, is not limited to thedescribed embodiments. Alterations and modifications to the disclosedembodiments may be made without departing from the invention. Themeaning of the terms used in the claims are, unless expressly statedotherwise, intended to have ordinary and customary meaning and are notintended to be limited to the details of the illustrated structures orthe disclosed embodiments.

What is claimed is:
 1. An apparatus comprising: a first member and asecond member, the first member coupled between a first and a secondjoint; the first joint being adapted for coupling to a base element topermit articulation of the first member with respect to the baseelement; a differential linkage adapted for receiving torque and fortransmitting torque to the first and second joints; a stop for limitingrotation of the second joint; and a linkage coupled between the baseelement and the stop for moving the stop based on the degree of rotationof the first joint.
 2. The apparatus of claim 1, further comprising aspring for placing a torque on the second joint in a direction thatforces the second member to extend with respect to the first member. 3.The apparatus of claim 2, wherein said stop for limiting rotation of thesecond, joint is connected to a stop linkage that moves said stop basedupon articulation of said first member relative to said base elementabout said first joint, wherein the spring comprises end point set bythe stop linkage.
 4. The apparatus of claim 3, wherein the second jointcomprises an axis of rotation, and wherein the stop is mounted forrotation about said axis of rotation of the second joint.
 5. Theapparatus of claim 1, further comprising a third joint and a thirdmember coupled to the third joint, the second member being coupledbetween the second joint and the third joint.
 6. The apparatus of claim5, further comprising a kinematic linkage for coupling rotation of thesecond and third joints.
 7. A mechanical finger comprising: a firstmember, a second member and a third member, the first member coupledbetween a first and a second joint, the second member coupled betweenthe second joint and a third joint, and the third member coupled to thethird joint; the first joint being adapted for coupling to a baseelement to permit articulation of the first member with respect to thebase element; a differential linkage adapted for receiving torque andfor transmitting torque to the first and second joints; a kinematiclinkage for coupling rotation of the second and third joints; a stop forlimiting rotation of the second joint; and a stop linkage coupledbetween the base element and the stop for moving the stop based on thedegree of rotation of the first joint.
 8. The apparatus of claim 7,further comprising a spring for placing a torque on the second joint ina direction that forces the second member to extend with respect to thefirst member.
 9. The apparatus of claim 8, wherein the spring comprisesan end point, and wherein the end point of the spring changes locationbased upon a location of the stop.
 10. The apparatus of claim 7, whereinthe stop is mounted for rotation about an axis common with the axis ofthe second joint.
 11. A prostheses comprising: a base portion; aplurality of fingers, each finger comprising a first member, a secondmember and a third member, the first member coupled between a first anda second joint, the second member coupled between the second joint and athird joint, and the third member coupled to the third joint; the firstjoint being adapted for coupling to a base element to permitarticulation of the first member with respect to the base element; adifferential linkage for receiving torque and for transmitting torque tothe first and second joints; a kinematic linkage for coupling rotationof the second and third joints; a stop for limiting rotation of thesecond joint; a stop linkage coupled between the first joint and thestop for moving the stop based on the degree of rotation of the firstjoint; and a spring for placing a torque on the second joint in adirection that forces the second member to extend with respect to thefirst member.
 12. The prosthesis of claim 11, further comprising amechanical arm, to which the base portion is attached.
 13. An apparatuscomprising: a first member and a second member, the first member coupledbetween a first and a second joint; the first joint being adapted forcoupling to a base element to permit articulation of the first memberwith respect to the base element; a differential linkage adapted forreceiving torque and for transmitting torque to the first and secondjoints; a spring for placing a torque on the second joint in a directionthat forces the second member to extend with respect to the firstmember; and a linkage coupled between the base element and an endpointof the spring for moving the endpoint based on the degree of rotation ofthe first joint.
 14. The apparatus of claim 13, further comprising athird joint and a third member coupled to the third joint, the secondmember being coupled between the second joint and the third joint. 15.The apparatus of claim 14, further comprising a kinematic linkage forcoupling rotation of the second and third joints.